Perpendicular magnetic tunnel junction with strained Mn-based synthetic ferrimagnets

摘要

The Mn-based alloys have been attractive attention as electrode materials for perpendicular magnetic tunneljunctions (p-MTJ) in magneto-resistive random access memory (MRAM) . This is because their highperpendicular magnetic anisotropy (PMA) and small magnetization is beneficial to shrink MTJ size towardincreasing recording capacity over 16 Gbits. Recently we developed the growth technique of ultrathin MnGafilms and MnGa-based p-MTJs with high PMA, however maximum TMR ratio were still below 20%.One of the solution for achieving both high TMR ratio and low magnetization is to use synthetic ferrimagnetstructure with CoFe-based insertion layer. In this presentation we propose new synthetic ferrimagnetscomposed of strained MnGa and CoFe-based insertion layer. The stacking structure is MgO(sub.)/Cr(40)/CoGa(30)/MnGa(3)/CoGa(t_(CoGa)=0, 0.4)/Fe(0.4)/MgO(2.4).CoFeB(1)/Ta(3)/Ru(5) (thickness is in nm), whichwere grown at room temperature using an ultrahigh-vacuum sputtering system. The micro-fabrication ofdevices was performed by a conventional photolithography and Ar ion milling. The MTJs were annealed atoptimized temperature so as to obtain maximum TMR ratio. Figure 1 shows the room temperature TMRcurve of the MTJs with t_(CoGa)=0 and 0.4 nm. Here, the MTJs with t_(CoGa)=0 was not annealed because theannealing at the temperature over 200℃ decreased the TMR ratio. The shape of the TMR curve indicatedferromagnetic coupling between MnGa and Fe layer, as reported in the thick-MnGa/Fe bilayer. On theother hand, the inverted TMR curve was observed in the MTJs with t_(CoGa)=0.4 nm, implying that the MnGaand Fe layers coupled antiferromagentically. In addition, the TMR increased up to 33% with the annealingat 250℃. These results suggest that the ultrathinCoGa spacer between the MnGa and the Fe layersinduces the antiferromagnetic coupling forsynthetic ferrimagnet and increases the thermalendurance of the Mn-based MTJs.This work waspartially supported by the ImPACT program andKAKENHI (17K14103).